The present invention relates to silyl phosphates and more specifically the present invention relates to silyl phosphates which are useful as improved neutralizing agents for alkali metal hydroxides in siloxane equilibration reactions.
Heat vulcanizable silicone rubber compositions as well as room temperature vulcanizable silicone rubber compositions are well known. In the case of heat vulcanizable silicone rubber compositions, the basic ingredients comprises triorganosiloxy endstopped linear diorganopolysiloxane polymer having a viscosity of anywhere from 1,000,000 to 200,000,000 centipoise at 25.degree. C. In the case of room temperature vulcanizable silicone rubber compositions the basic ingredient comprises a silanol endstopped linear diorganopolysiloxane polymer or a vinyl terminated linear diorganopolysiloxane having a viscosity of anywhere of 300 to 500,000 centipoise at 25.degree. C. In the case of heat vulcanizable silicone rubber compositions, the triorganosiloxy endstopped diorganopolysiloxane polymer is usually prepared by equilibrating cyclic tetrasiloxanes in pure form at elevated temperatures, that is temperature above 100.degree. C. in the presence of 5 to 500 parts per million of alkali metal hydroxide. The cyclic tetrasiloxanes that are utilized in such reactions are usually obtained by first hydrolyzing diorganodichlorosilanes in water and then cracking the hydrolyzate with an alkali metal hydroxide to obtain a mixture of cyclopolysiloxanes followed by a distillation to obtain the cyclic tetrasiloxanes.
In the above, the organo groups can be any monovalent hydrocarbon radical. In the case of the silanol endstopped diorganopolysiloxane polymers, such polymers are obtained by equilibrating cyclictetrasiloxanes or mixtures of cyclopolysiloxanes or a siloxane hydrolyzate at elevated temperatures, temperatures above 100.degree. C., in the presence of small amounts of water or with small amounts of low molecular weight silanol endstopped diorganopolysiloxane polymers. The reaction is carried out in the presence of small amounts of alkali metal hydroxides until the desired viscosity polymer is obtained. In an alternate process, low viscosity silanol endstopped diorganopolysiloxane polymers are obtained by equilibrating pure cyclictetrasiloxanes or mixtures of polysiloxanes or siloxane hydrolyzates in the presence of acids such as toluenesulfonic acid or acid activated clay such as Filtrol sold by Filtrol Corporation of Los Angeles, California.
In the case of fluorosilicone polymers, such equilibration reactions can be carried out with facility with cyclictrisiloxanes in the presence of alkali metal hydroxides, most preferably sodium hydroxide. With such catalysts, there results the maximum production of linear polymer and also such catalysts belong to the class of the few catalysts that can be used in the production of high viscosity linear diorganopolysiloxane polymers. For the preparation of fluorosilicone polymers from cyclictetrasiloxanes, see for instance Razzano U.S. Pat. No. 3,997,496 and U.S. Pat. No. 3,937,684.
Going further into the equilibration reaction of cyclictrisiloxanes with the alkali metal hydroxides when approximately 85% (which is the maximum that can be converted of the cyclictetrasiloxane to the polymer) of the cyclictetrasiloxanes have converted to the polymer the catalyst is neutralized with an acidic material and the excess cyclics are distilled off to yield the desired linear diorganopolysiloxane polymer which can be utilized as a basic ingredient either in the heat vulcanizable silicone rubber compositions or in room temperature vulcanizable silicone rubber compositions depending on the viscosity of the polymer and whether it is silanol terminated or not.
Over the years various neutralizing agents have been tried for such alkali metal hydroxides, and alkali metal silanolates catalysts that are present in the equilibration mixture, after the equilibration has been reached. Such acids as hydrochloric, acetic, phosphoric and toluenesulfonic acid as well as other acids have been tried for neutralization of the alkali metal hydroxide catalyst. All of these neutralizing agents work with varying success. For instance, phosphoric acid is highly desirable in that it is a triprotic acid, which, when it is used in neutralizing an alkali metal hydroxide forms potassium dihydrophosphate, dipotassium hydrophosphate and tripotassium phosphate. The advantage of phosphoric acid is that the potassium dihydrophosphate and dipotassium hydrophosphate act as buffering agents in the system, which helps to protect the polymer from traces of acidic or basic impurities and is tolerant of small errors in adding the theoretical amount of phosphoric acid neutralizing agent. In the case of the other types of acidic neutralizing agents such as for instance toluenesulphonic acid, it was necessary to determine the exact amount of acid needed to neutralize the alkali metal hydroxide such that there would not be large residual amounts of acid left in the system which would degrade the linear polymer and finally the product in which it was incorporated. Accordingly, in such neutralization procedures, it was common to add the exact amount of acid and test the mixture for its pH and adjust the alkali metal hydroxide content or acid content to as close as neutral as was possible within the time limits of manufacturing production.
Accordingly, since phosphoric acid was a triprotic acid, it was highly preferred as a neutralizing agent for alkali metal hydroxides in the equilibration of siloxanes. However, phosphoric acid has one disadvantage, it is insoluble in siloxanes and as such would require constant agitation for long periods of time to obtain the proper mixing of the acid into the equilibration siloxane mixture. In a 50 gallon reaction kettle having therein a moderate viscosity siloxane mixture, it requires with moderate agitation 20 minutes for the phosphoric acid to blend into the siloxane mixture for neutralization purposes and with good agitation it requires 8 minutes. Accordingly, it should be noted that with the high viscosity high volume diorganopolysiloxane mixtures it would require excessive time for the blending of the phosphoric acid in this diorganopolysiloxane mixture.
It should also be noted that there are other triprotic acids such as for instance arsenic acid. However, this material is highly toxic as compared to the relatively non-toxic phosphoric acid, and also arsenic acid has undesirable reduction and oxidizing properties.
Accordingly, for one reason or another other triprotic acids have one disadvantage or another. As stated previously, phosphoric acid is readily available at low cost, is relatively non-toxic and has the desired buffering action in the neutralization of alkali metal hydroxides. There has also been developed a continuous process for the production of silanol endstopped diorganopolysiloxane polymers. Accordingly, phosphoric acid appears to be highly desirable as a continuous neutralizing agent for such continuous equilibration reactions. However, as stated previously, phosphoric acid has the disadvantage in that it is considerably insoluble in such siloxanes, and as such cannot be used in such continuous equilibration reactions since it would not blend into the siloxane equilibration mixture in the time allotted to carry out the desired neutralization and require complex mixing equipment. Accordingly, it was highly desirable to find a soluble form of phosphoric acid which would be utilized for the continuous equilibration neutralization of siloxane mixtures containing alkali metal hydroxides.
In the past few years there has been developed a soluble phosphate which is soluble in a siloxane mixture by reacting phosphoric acid and octylmethylcyclictetrasiloxanes with small amounts of chainstopper such as hexamethyldisiloxane. The resulting silyl phosphate had good neutralizing properties for siloxane equilibration reaction mixtures. However, such a material had two disadvantages, the most important disadvantage being that the viscosity of the silyl phosphate was above 500 centipoise at 25.degree. C. making it difficult to blend into the siloxane equilibration reaction mixture. A minor disadvantage is that in the production of such silyl phosphates there could only be obtained silyl phosphates which had a phosphoric acid content at a maximum of 10 to 15% by weight. Accordingly, if there was desired to obtain a silyl phosphate with a higher phosphoric acid content it was not possible to obtain such a product using the prior art process. It was desired to obtain a silylphosphate having a phosphoric content of 20 to 30% by weight because such high phosphoric acid content silylphosphate would be desirable for fast neutralization of siloxane equilibration reaction mixture in continuous polymerization process.
Accordingly, it was highly desirable to have a process and a product that is a silyl phosphate which did not have the above disadvantages.
Accordingly, it is one object of the present invention to provide for a silyl phosphate which is obtained by reacting phosphorous oxyhalogen with a linear diorganosiloxane.
It is an additional object of the present invention to obtain a silyl phosphate which is obtained by reacting phosphoric acid with a linear diorganosiloxane.
It is an additional object of the present invention to provide for silyl phosphates which can have a phosphoric acid content of anywhere of 5 to 30% by weight and more preferably 20 to 30% by weight and a viscosity below 500 centipoise at 25.degree. C.
It is still another object of the present invention to provide for a silyl phosphate which is imminently suitable in the continuous neutralization of alkali metal hydroxides in the continuous production of siloxanes.
It is still another object of the present invention to provide for a process for producing novel silyl phosphates, that generally can have a phosphoric acid content of anywhere from 5 to 30% by weight and a viscosity less than 500 centipoise at 25.degree. C. These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.